Chicago City Hall rooftop garden

Chicago’s most famous rooftop garden sits atop the City Hall, an 11-storey office building in the Loop. The City Hall and the adjacent Cook County buildings appear to most people as one building spanning a city block bounded by LaSalle, Randolph, Clark and Washington streets. Initiated in 2000, the City Hall rooftop garden was conceived as a demonstration project—a part of the City’s Urban Heat Island Initiative—to test the benefits of green roofs and how they affect temperature and air quality. The garden consists of 20,000 plants of more than 150 species, including shrubs, vines and two trees. The plants were selected for their ability to thrive in conditions on the roof, which is exposed to the sun and can be windy and arid. Most are prairie plants native to the region.

Like all green roofs, the City Hall rooftop garden improves air quality, conserves energy, reduces storm water runoff and helps lessen the urban heat island effect. The garden’s plants reflect heat, provide shade and help cool the surrounding air through evapotranspiration, which occurs when plants secrete or transpire water through pores in their leaves. The water draws heat as it evaporates, cooling the air in the process. Plants also filter the air, which improves the air quality by using excess carbon dioxide to produce oxygen.

The rooftop garden mitigates the urban heat island effect by replacing what was a ballasted, black tar roof with green plants. The garden absorbs less heat from the sun than the tar roof, keeping the City Hall cooler during summers and requiring less energy for air conditioning. The garden also absorbs and uses rainwater. It can retain 75% of a 1-inch rainfall before there is stormwater runoff into sewers.

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Chicago City Hall rooftop garden

Chicago’s most famous rooftop garden sits atop the City Hall, an 11-storey office building in the Loop. The City Hall and the adjacent Cook County buildings appear to most people as one building spann...